s = input()
mx = 0
b = [s[0], s[1], s[2], s[0:2], s[1:3], s[2:4], s[0:3], s[1:4], s]
for h in range(len(b)):
    a = int(b[h])
    c=0
    for i in range(1, a+1):
       if a%i == 0:
           c = c + 1
    if c ==2:
         mx = max(mx,a)
print(mx)


    
